Cost effective optical transmission with fast raman tilt transient control

ABSTRACT

A method for cost-effective optical transmission with fast Raman tilt or other transient event control uses a combination of Erbium-doped fiber amplifiers (EDFAs) and Raman fiber amplifiers (RFAs), where EDFAs are used as the primary optical amplifiers to compensate the span loss while the RFA (advantageously a forward-pumped RFA) is used only in some specific spans with a feed-forward control circuit serving as a fast Raman tilt transient compensator, the RFA also serving as an optical amplifier. A long haul optical transmission system using feed-forward controlled RFA&#39;s periodically spaced along its length, for example, when add-drop multiplexing is used, makes full use of the economics of EDFAs and the fast tilt transient control capability of a RFA enabled by an adjustable speed feed-forward or feed-back control technique.

This application is a continuation application of Ser. No. 11/424,307,filed Jun. 15, 2006, which issued as U.S. Pat. No. 7,446,932 on Nov. 4,2008.

FIELD OF THE TECHNOLOGY

This invention relates to the field of long haul, multi-channel fiberoptic transmission systems, and, more particularly, to a system in whicha transient event in such systems, especially a tilt transient, iscontrolled by the periodic application of Raman amplifiers withfeed-forward or feedback gain control in a hybrid rare earth doped/Ramanoptical amplifier end-to-end transmission system.

BACKGROUND

Amplification in optical fiber transmission systems using rare earthdoped optical fiber amplifiers, such as, for example, erbium doped fiberamplifiers (EDFA) has been implemented widely due to the advantageouseconomics and wideband multi-channel operation of such amplifiers. Insuch an optical transmission system, inter-channel stimulated Ramanscattering (SRS) may result in a tilted gain characteristic across thewavelength division multiplexed (WDM) channels (denoted as Raman tilthereafter) by externally supplied Raman pump radiation using the Ramanpumps to add Raman gain. Tilt is the well known transmission impairmentcharacterized by increased power consumption at decreasing wavelength(increasing frequency). Without compensation for tilt, such an effectaccumulates span by span and results in serious optical signal to noiseratio (OSNR) degradation in the shorter-wavelength channels and seriousnonlinear penalty in the longer-wavelength channels. In a traditionalpoint-to-point WDM system, it is known to compensate for Raman tilt dueto inter-channel SRS in the transmission fiber by adding a static tiltcompensator after every span. But optical communication is evolving fromcurrent point-to-point systems (in which there are no intermediateadd/drop points) to dynamic optical networks, in which channels will beadded and dropped at intermediate points in the end-to-end system byusing, for example, known remotely reconfigurable optical add/dropmultiplexers (ROADM) to meet the varying capacity demands. A typical 80channel C-band WDM system using a single mode fiber as the transmissionfiber of, for example, a span length of twenty spans can contribute 1 dBof tilt per span across C-band or a cumulative 20 dB of Raman tilt willbe present in such a long haul system. (Raman tilt can be even higherwhen nonzero-dispersion shifted fibers are used as the transmissionfibers).

FIG. 1 is an illustration of an exemplary fiber optic amplifier systemhaving a transmission fiber (Trans. Fiber) 100-1 followed by a firstErbium doped fiber amplifier (EDFA). In the drawings, similar referencenumerals will be used to denote similar elements and the first number ofa given reference numeral indicates the number of the figure where thatelement first appears. A dispersion compensation fiber (DCF) 102separates the first EDFA 101-1 from a second Erbium doped fiber opticamplifier 101-2. A reconfigurable optical add/drop multiplexer (ROADM)103 is shown following the second EDFA 101-2 for adding and droppingchannels. Adding and dropping channels causes transient events, forexample, tilt transients. These events, if left uncompensated, createtransmission degradation. Depending on the wavelengths dropped andadded, a fast transient event occurs, typically, in the form of atransmission degrading tilt transient. Referring to FIG. 1, also,accidental loss of channels due to a transmission fiber cut or acomponent failure of one of the components such as an EDFA or the DCF102 in front of an ROADM 103 will also lead to a sudden change inchannel count and transient events such as tilt transients. Therefore,there will be a resulting change of the overall optical power in a linkfollowing an ROADM node. Of course, the purpose of an ROADM is to addand to drop channels which can likewise result in a change of overalloptical power. The strong reduction/increase of total launch power intothe transmission fiber section (Trans. Fiber) 100-2 following the ROADM103 will result, for example, in a substantial reduction/increase ofRaman tilt.

As is shown in FIG. 1, the respective channel patterns 1 and 2 shownafter ROADM 103 will exhibit tilt in their spectral powers across theirbandwidth as represented by the same channel patterns depicted at theoutput of Trans. Fiber 100-2. The depicted tilt is represented as asmooth sloping line as a general case to show Raman tilt. In the case ofco-directional propagation of the signal channels, the transition timeof the induced transients are equal to the transition time of theswitching events, which can be very fast. As a result, an opticaltransmission system with fast Raman tilt transients control capabilityis needed for operating in a dynamic optical network. A “tilt transient”is not shown by way of example in FIG. 2( b) where the composite Ramangain profile results in a tilted profile but, depending on the droppedor added wavelengths or the cause of a transient event, the transientevent may cause bumps (overshoots and undershoots) in the smooth tiltgain characteristic and thus may be described generally as a transientevent and is not one as depicted that specifically results in a smoothlytilted gain profile.

Recently, P. M. Krummrich and inventor Martin Birk, in their article“Compensation of Raman Transients in Optical Networks,” presented at OFC2004, paper MF 82 and their article, “Experimental Investigation ofCompensation of Raman-induced Power Transients from WDM ChannelInteractions,” IEEE Photonics Technology Letters, Vol. 17, no. 5, pp1094-96, May, 2005, suggest adding a standalone dynamic tilt compensatorafter every span or after a small number of spans into a traditionalEDFA system to deal with this problem. In their method, the dynamic tiltcompensator is based on a periodically poled LiNbO3 technology with atwo-stage design. But such a solution adds considerable cost to the costof a long haul transmission fiber optic system because the fast dynamictilt compensator itself is quite expensive. Moreover, one dynamic tiltfilter may introduce more than 5 dB of insertion loss into the system.Therefore, the insertion loss of the device will require additionalamplification to compensate for the insertion loss which, if theamplification is necessary, will add to the overall system cost.

Also, recently, the inventors have prepared and are filing a number ofpatent applications directed to dynamic gain control for a fiber opticsystem as represented by U.S. patent application Ser. Nos. 11/273,868and 11/274,666 filed Nov. 15, 2005; U.S. patent application Ser. No.filed (Attorney Docket No. 2004-0532 CIP; 003493.00496); and U.S. patentapplication Ser. No. 11/381,244, filed May 2, 2006 (Attorney Docket No.2005-0617; 003493.00495), all incorporated by reference as to theirentire contents. For example, the inventors propose the use of opticalamplifiers which are either forward or reverse pumped, RFA's comprisinga plurality of Raman pumps that may be controlled by a single controlcircuit and feed forward and feed backward control circuits andequations and algorithms for their control in combination or usedseparately. In the specification and claims, a forward Raman pump is apower source that provides power to a signal by a co-propagatingsignal-pump Raman interaction and a backward Raman pump is a powersource that provides power to a signal by counter-propagatingsignal-pump Raman interaction. A Raman fiber amplifier, either forwardor reverse, and an associated dynamic gain control circuit caninherently provide transient tilt control because of their inherentspeed.

Apart from a pure EDFA system (no Raman amplification), a hybrid (orcombined) end-to-end fiber optical system which includes both EDFA andRaman amplifiers in the system or an all-Raman system have also beenwidely investigated in recent years. In a known hybrid EDFA/Ramanoptical system, both EDFA and Raman fiber amplifiers (RFA) (distributed)are used in many or even every span. In an all-Raman system, RFAs(distributed and discrete) are the only optical amplifiers used. In bothsystems, externally supplied Raman pumps are fed into the transmissionfiber 100 at every span; consequently, the signal experiences Raman gainnot only from the other signals input to the system but also from theRaman pumps. Because usually more than one Raman pump is needed toobtain a flat gain characteristic over a wide bandwidth and because thepower conversion efficiency of a distributed RFA is typically lower thanan EDFA, using a distributed RFA at every span results in a considerablecost increase compared to an EDFA only system. Such a cost penalty mayoutweigh the noise performance gain provided by using distributed RFAsat every span under some circumstances. Consequently, there remains aneed in the art for a cost effective approach to the problem oftransient event control, especially, tilt transients, due, for example,to channel addition and dropping in a long haul fiber optic system.

SUMMARY

The problems of controlling transient events such as tilt transients inan end-to-end long haul fiber system can be solved by applying afeed-forward gain control technique at a Raman fiber amplifier (RFA)periodically placed in a long haul hybrid or Raman system. Inparticular, the detrimental tilt transients due to inter-channel SRS ina Raman-amplified WDM system can be suppressed by using an existingmulti-wavelength-pumped RFA with a feed-forward dynamic gain controlequation and external devices that provide transient event compensationspeed adjustment. This is due to the facts that 1) the gain profile of amulti-wavelength-pumped RFA can be easily reconfigured by adjusting thepower distribution of different Raman pumps (as will be discussed withreference to FIG. 2); 2) there exist substantial linear relationships ina multi-wavelength-pumped RFA which can be used to simplify theprocedure to obtain the proper pump power adjustments under variouschannel loading conditions, and 3) the response time of an RFA is fasterthan that of any rare earth doped fiber amplifier such as an EDFA. (Aforward-pumped RFA has an almost identical response time as theinter-channel SRS effects, but the response time of an EDFA is muchslower than inter-channel SRS effects). Moreover, as the gain equationwill demonstrate and according to one embodiment, transient event speedadjustment is provided within the dynamic gain control circuit itself,for example, by determining the propagation time between detection of atransient event and its resolution and incorporating the result into thecircuit design via the gain equation. Moreover, in an alternativeembodiment, transient event compensation speed may be adjusted using aknown fast spectral information monitor in a feed-forward control path.Moreover, a fast channel monitor may be utilized in a feedback controlpath. Other embodiments will be discussed herein using opticalsupervisory channels and slow spectrum monitors in a feedback path.

As a result of these investigations and investigations into configuringa hybrid EDFA/Raman amplifier system, an EDFA/Raman hybrid system can bedesigned to achieve both cost-effective optical transmission as well asfast Raman tilt transient control. According to one aspect, acombination of EDFA and RFA are used in only a small number of spanswhile EDFAs are used as the predominant optical amplifiers in most ofthe spans. After several EDFA-only spans, for example, it is suggestedto use an RFA (advantageously a forward-pumped RFA) to replace one ofthe two EDFAs in the following span (two EDFAs per span are required fora typical EDFA system) to perform fast tilt transient control as well asoptical amplification. Each RFA is used to control the overall tilttransients generated from multiple spans (advantageously three to sixspans). The acceptable accumulated tilt depends on system designcriteria such as margin allocation and total span number, but usuallyshould not be greater that 4 or 5 dB for an ultra-long haul WDM system.An introduced RFA also performs the function of the replaced EDFA as anoptical amplifier, so such as transmission system is designed to be morecost-effective than systems suggested by Krummrich and others by addingstandalone dynamic tilt compensators into an EDFA-only WDM system. Thisis especially the case if an existing dispersion compensation fiber(DCF) 102 is utilized as the gain medium to construct a discrete RFArecognizing that the DCF 102 has a higher Raman gain coefficient than atypical transmission fiber 100. Consequently, the power conversionefficiency is improved in such a discrete RFA. Moreover, the introducedRFA can also perform a dynamic gain equalization (DGE) function whichmay eliminate the need or at least reduce the number of standalone DGEsrequired for an ultra-long-haul (ULH) WDM system. Reducing the use ofstandalone DGE's also contributes to cost reduction.

These and other features of a hybrid rare earth doped and/or Raman fiberamplifier only long haul fiber optic amplifier system with transienttilt control will become clear from the drawings and the detaileddescription thereof which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram depicting a multi-span long haulfiber optic transmission system for carrying a plurality of channels inpatterns including, primarily, rare earth doped fiber amplifiers such aserbium doped fiber amplifiers (EDFA); the drawing is used to definetransient tilt which can dynamically occur due to fiber breaks,component failures and the use of add/drop multiplexers to add and dropchannels during long haul transmission.

FIG. 2( a) represents a composite Raman gain profile from a plurality ofchannels in a long haul fiber optic transmission system where there isno transient tilt introduced, for example, by adding or droppingchannels and static or dynamic tilt compensation is employed resultingin a flat gain profile; FIG. 2( b), on the other hand, demonstratesdifferent energy at different channels and a tilted composite Raman gainprofile results, for example, when no dynamic gain control is providedin cooperation with periodically placed Raman fiber amplifiers (RFA's).

FIG. 4 shows graphs representing calculated static optical powerdeviation per channel after four spans of transmission where FIG. 4( a)represents a graph showing results for the proposed system of FIG. 3with only a feed-forward circuit enabled and FIG. 4( b) represents apure EDFA system without any Raman transient or static tilt control.

FIG. 5 shows a second embodiment of the invention utilized in anexemplary four span system using a backward-pumped discrete RFA (Ramanpumps) in the second span with a feed-forward dynamic gain controlcircuit (the feed-forward signals being remotely monitored) to controlthe overall Raman tilt transients from multiple spans.

FIG. 6 shows a third embodiment of the invention using a forward-pumpeddiscrete RFA (Raman pumps) in the second span with a feed-forwarddynamic gain control circuit (feed-forward signals being monitored rightbefore the RFA) to control the overall Raman tilt transients frommultiple spans; in this embodiment a fiber delay line (FDL) isintroduced in the second span for fast signal monitoring which may be ahybrid span and a DCF is shown after the WDM.

FIG. 7 shows a fourth embodiment of the invention using abackward-pumped discrete RFA (Raman pumps) in the second span with afeed-forward dynamic gain control circuit (feed-forward signals beingmonitored right before the RFA) to control the overall Raman tilttransients from multiple spans; in this embodiment, a DCF is shownbefore the WDM optical circulator.

FIG. 8 shows a fifth embodiment of the invention using a forward-pumpeddiscrete RFA (Raman pumps) in the third span with a feedback-baseddynamic gain control circuit to control the overall Raman tilttransients from multiple spans.

FIG. 9 shows a sixth embodiment of the invention using a backward-pumpeddiscrete RFA (Raman pumps) in the third span with a feedback-baseddynamic gain control circuit to control the overall Raman tilttransients from multiple spans.

FIG. 10 (a)-(d) show block schematic drawings for four exemplary longhaul WDM systems and, in particular, placements for an EDFA/Raman spanin which one of the embodiments of FIGS. 3 and 5-9 or equivalentembodiments may be employed and the relation of such a span to thelocation of an ROADM; a multitude of alternative embodiments for placingan ROADM differently in relation to an EDFA/RAMAN span are possible.

DETAILED DESCRIPTION

As introduced above, the problems of providing dynamic tilt transientcontrol in a hybrid or Raman only long haul fiber optic system can besolved by introducing periodically a feed-forward control circuit at thelocation of a Raman amplifier as will be further described withreference to FIGS. 3 through 10 where FIG. 2 shows the impact ofadjusting the power distribution of different Raman pumps (RFA's). Inparticular, in FIG. 2 (a), there is shown a flat composite gain profilewhich is ideally achieved and in the presence of no transient events.According to FIG. 2( b), a tilted composite gain profile is shown by wayof example of a transient event that may occur when there is a fiber cutor an add/drop multiplexer event or other component failure. The tiltshown in FIG. 2( b) is shown as a sloping straight line where in realitythe line may include one or more overshoots or undershoots depending onthe triggering transient event.

FIG. 3 shows a first embodiment of the invention, in which aforward-pumped discrete RFA (Raman pumps) 300 further comprises afeed-forward dynamic gain control circuit including span 1 circuit 301-1and span 2 circuit 301-2 connected by an optical supervisory channel tocontrol the overall Raman tilt transients generated from multiple spansof transmission fiber and a dispersion compensation fiber (DCF) 102-1 to102-4 utilized in each span. In this specific example, four transmissionspans following a ROADM 103 are shown. The forward-pumped RFA 300 isshown located at the second span. In an alternative embodiment, thecontrolled RFA 300 may also be placed in the first span, the third spanor the final span as will be further discussed in relation to FIG. 10.Placing the RFA (Raman pumps) 300 in a middle span (i.e. the second spanin this example) is an appropriate choice because the noise andnonlinear penalty due to Raman gain tilt may be minimized in such aconfiguration. For the fast (in the magnitude of microseconds)feed-forward gain control circuit 301-1, 301-2, a small part of theinput WDM signal is tapped out at a Coupler before it enters into thetransmission fiber (Trans. Fiber) 100-1 of the first span (of the fourspans). The tapped-off signal shown from the Coupler is then showndivided into K wavelength regions, S₁ through S_(k), via a bandwavelength-division multiplexer (B-WDM) where it enters a first ControlUnit 301-1. The total power in each of the K wavelength regions is thendetected by a respective photodetector (PD) (to monitor the input signalspectral information). Simulation results have shown that K=2 (tappingoff two signal wavelengths) is usually adequate for a typical C/L bandWDM system, but K can be chosen to be 1 or greater than 2 depending uponthe system requirement. The information about the total input power ineach wavelength region, S₁ . . . S_(K), is then sent to the secondControl unit 301-2 for the RFA (Raman pumps) 300 located at the secondspan through a common optical supervisory channel (OSC). (A time delay,(not shown in this embodiment) may be introduced in the normaltransmission path by inserting a fiber delay line (FDL) in this path tocompensate for the required processing time of sending a feed-forwardsignal in the OSP path). During channel add/drop (caused by device orfiber failures or the add/drop activity of ROADM among other reasons),the power of each of the Raman pumps shown collectively as pumps 300 maybe adjusted using the following linear equation

$\begin{matrix}{{P_{L}( {j,t} )} \approx {{P_{L\; 0}(j)} + {\sum\limits_{k = 1}^{K}{{T_{LL}( {j,k} )}\lbrack {{S_{L}( {k,{t - T}} )} - {S_{L\; 0}(k)}} \rbrack}}}} & {{EQ}.\mspace{14mu} 1}\end{matrix}$

where P_(L) (j,t) denotes the required pump power in the linear unit ofthe j^(th) pump at time instant t, S_(L) (k,t) denotes the detectedinput signal power in the k^(th) wavelength region also in the linearunit. S_(L0)(k) and P_(L0)(j) denotes the corresponding input signalpower and pump power at the reference operation point, e.g. the casewith a full load or with a half load. T denotes a time delay between therequired pump power adjustment for the discrete RFA and the detectedinput signal power variation at the monitoring point. Ideally T shouldbe equal to the propagation time of the signal from the monitoring pointto the signal-pump WDM located at the second span. T_(LL) (j,k) denotesthe linear control coefficient. By way of example and as shown in FIG.3, T is the measured from the monitoring point location of Coupler tothe WDM shown in Span 2. Consequently, transient event compensationspeed may be adjusted by using an estimation of T depending onpropagation times for intermediate components and the fiber delaybetween Coupler and WDM. Also, for a specific system design, linearcontrol coefficient T_(LL)(j,k) uniquely depends on the passive opticallink parameters such as fiber length, fiber loss and Raman gaincoefficient, and therefore can be predetermined either by directmeasurement or by numerical simulation using the measured basic opticallink parameters.

A simple method for determination of linear control coefficientT_(LL)(j,k) is given as follows. First, we configure the signal channelsin such a way that only in the kth wavelength region the detected signalinput power is different from a reference point and thenmeasure/calculate the required power adjustment of each Raman pump (tomaintain the signal power level per channel at the output of the fourthspan to the target level). Let ΔP_(L) (j) denote the required staticpower adjustment of pump j of Raman pumps 300, and ΔS_(L)(k) thedetected static input signal power variation in the kth wavelengthregion, then we can determine T_(LL)(j,k)=ΔP_(L)(j)/ΔS_(L)(k).

Apart from the above-mentioned fast feed-forward tilt transient controlcircuit 301-1, 301-2, a relatively slow (magnitudes of milliseconds orseconds) feedback loop may be also introduced to perform a conventionaldynamic gain equalization (DGE) function to reduce the gain ripple thatmay be accumulated from cascaded EDFAs 101 and polarization-relatedissues. For this feedback loop, a small part of the output signal poweris coupled out at the output of the fourth span and fed back through arelatively slow power spectrum monitor. Such a slow power spectralmonitor typically consists of a full-band tunable filter followed by anoptical power detector. The output channel power spectral information isthen sent back to the control unit 301-2 of the RFA 300 as a feedbacksignal to control each of the Raman pumps of pumps 300. The conventionalproportional-integration-derivative (PID) algorithm or an iterativemethod based on direct Raman power simulation may be used as the controlalgorithm for control unit 301-2. A PID(proportional-integral-derivative) based feedback pump power controltechnique is described, for example, by C. J. Chen et al., “Control ofTransient Effects in Distributed and Lumped Raman Amplifier,” ElectronicLetters, pp. 1304-05, October, 2001; L. L. Wang et al., “Gain Transientsin Co-pumped and Counter-pumped Raman Amplifiers,” IEEE PhotonicsTechnology Letter, pp. 664-666, May, 2003, and M. Karasek et al.,“Modeling of a Pump-power-controlled Gain-locking System for multi-pumpWideband Raman Fiber Amplifiers,” IEEE Proceedings—Optoelectronics, pp.74-80, April, 2004, P. M. Reepschlager et al. (EP 1248334), and C. J.Chen et al. (U.S. Pat. No. 6,441,950) incorporated herein by referenceas to their entire contents. This slow feedback gain control loop canalso help in reducing residual static error that may accumulate by usinga purely feed-forward control technique and help in automaticmeasurement of the linear control coefficient T_(LL) (j,k) that isrequired for the forward-forward control circuit 301-1, 301-2, becausethe required pump power adjustment for different channel patterns can beautomatically obtained by using this slow feed-back gain control loopincluding the slow power spectral monitor. For an ultra-long-haul WDMsystem with more than one RFA 300 are used, the pump wavelengths atdifferent RFAs may be chosen to be different to enhance the capabilityof overall gain ripple suppression.

Referring to FIG. 4, FIG. 4 (a) numerically demonstrates theeffectiveness of the proposed Raman tilt control technique as is shownin the first embodiment of the invention depicted in FIG. 3. FIG. 4assumes a four-wavelength (1425, 1436, 1452 and 1466 nm) forward-pumpeddiscrete RFA located at the second span of an EDFA system, the RFA beingused to compensate the overall Raman tilt generated from the four spansof transmission for an 80-ch 50 GHz-spaced WDM system with input signalpower 2 dBm per channel. In each of the depicted four spans, forexample, 80 km of standard single mode fiber (SSMF) may be used as thetransmission fiber. The DCF 102 for each span may comprise 12 km of DCFas the dispersion compensator. To focus on the Raman tilt, we assumethat the depicted EDFA 101 (two of which are shown in each of spans 1, 3and 4) has an ideal flat gain profile. For the EDFA-only spans, thegains from the first EDFA and from the second EDFA are assumed to be 11dB and 13 dB, respectively (span loss+DCF insertion loss=23 dB). For thehybrid EDFA/Raman span 2, the gain from the single EDFA 101-3 is 13 dBwhile the gain from the RFA (Raman pumps 300) varies with the signalpatterns. In simulations, polarization-averaged Raman gain coefficientis used to simulate both inter-channel SRS and signal-pump SRS effects.For the feed-forward control circuit 301-1, 301-2, the input signal isdivided into two (K='s 2) wavelength regions (1529-1545 nm) and(1545-1561 nm), and we define the full load case (80 channel signals) asthe reference point. The 8 linear control coefficients are decided byusing two predetermined channel patterns (with only the first 40channels and with only the final 40 channels) with the proceduredescribed in the prior section. The four linear control equations for anassumed four Raman pumps 300 are found to be

P(1)=130−0.6675ΔS(1)−0.33ΔS(2)  EQ. (2)

P(2)=98.0−0.5150ΔS(1)−0.35ΔS(2)  EQ. (3)

P(3)=37.4−0.115ΔS(1)+0.005ΔS(2)  EQ. (4)

P(4)=24.5+0.5975ΔS(1)+0.76ΔS(2)  EQ. (5)

-   Where P(1), P(2), P(3) an P(4) denote the required pump power (mW)    for the four Raman pumps, and ΔS(1) and ΔS(2) denote the detected    input signal power variations (mW) in the two wavelength regions.

FIG. 4 (a) gives the calculated static power deviation per channel afterfour spans of transmission for 11 distinctive channel patterns based onthe above four linear control equations (pattern 1 is the full loadcase; for the other 10 patterns, the surviving channels range from 1channel to 60 channels, but the two patterns used for determination ofthe linear control coefficients are not included in the 10 patterns).One can see that for all the 11 patterns, the maximum power deviationsare suppressed to below 0.35 dB by using a purely feed-forward controltechnique. As a comparison, in FIG. 4 (b), we show the calculated powerdeviations for a purely EDFA system without Raman tilt control. One cansee that the maximum power deviation can be up to 3 dB (tilt up to 5.5dB) after four spans of transmission. As suggested earlier, in a longhaul twenty span system, tilt may be on the order of 20 dB across thespectrum.

In the above four-span example, we use one discrete RFA (Raman pumps300) to control the overall tilt transients and the feed-forward signal(input signal spectral information) is monitored at the input of thefirst span. For an ultra-long-haul WDM system with many spans (10 ormore), more than one RFA 300 are typically required. For this long haulcase, a feed-forward signal monitor may be shared by multiple RFAsfollowing the same ROADM 103, i.e. the fast spectral informationmonitors are only placed in the ROADMs 103 and the monitored outputspectral information (out of the ROADM but input of the transmissionfiber) is then sent to the downstream RFAs (before another ROADM) as thefeed-forward control signal.

In FIG. 3, we show a forward-pumped discrete RFA 300 to compensate theRaman tilt. As an alternative, a backward-pumped discrete RFA (Ramanpumps 300) may also be used to compensate the Raman tilt as isillustrated in FIG. 5 where OC represents an optical circulator locatedafter the DCF 102-2 of the second span. The method shown FIG. 5 exhibitsa slower control speed than the method shown in FIG. 3 but is morerobust to polarization-related issues.

In FIG. 6 and FIG. 7, there are shown another two embodiments of theinvention, where the feed-forward signals are monitored right before theRFA 300 in the second span by using a fast spectral information monitor.The fast spectral information monitor may be the same as that shown inFIG. 3 (monitoring the total signal powers in several wavelengthregions), but it may also be a fast channel monitor in which the signalpowers in several individual wavelength channels are monitored. For thecase with a forward-pumped RFA (Raman Pumps) 300 (according to FIG. 6),a short fiber delay line FDL (for example, of a few hundreds of meters)with a time delay equal to the time delay introduced by the controlbranch may be introduced in the transmission branch to enhance thecontrol speed. During channel add/drop or other cause of transient tilt,the power of each of the Raman pumps 300 may also be adjusted using thelinear equation of Equation 1 given above. For these two cases,S_(L)(k,t) may denote the detected input signal power in the k^(th)wavelength region or in the k^(th) signal channel. For the embodiment ofFIG. 6, T≅0. But T is approximately equal to the propagation time in theDCF 102-2 for the embodiment of FIG. 7.

When the span loss is large or the DCF length is short, the gainprovided by the RFA (Raman pumps 300) may not large enough to completelyreplace one of the EDFA 101. In this case, two EDFA's may be used in thehybrid EDFA/Raman span such as span 2; (the required gain from one ofthe two EDFAs is smaller than that required in an EDFA-only span). Forthe case that there is no DCF 102 in the transmission span (e.g. a fiberBragg grating (FBG, not shown) is used as the dispersion compensator) orthe DCF length is short, a distributed RFA (using the transmission fiberas the gain medium) or a combination of a distributed RFA and a discreteRFA with the same feed-forward dynamic gain control circuit as is shownin FIGS. 3, 5, 6 and 7 may also be used to perform Raman tilt transientcontrol. In addition, although the proposed feed-forward controltechnique is advantageously used to control the fast Raman tilttransients, the traditional feedback-based control method (such asPID-based control method) with a fast channel monitor may also be usedto control the Raman tilt transients depending on specific applicationsand system requirements as is shown in the respective embodiments of theinvention shown in FIGS. 8 and 9. Furthermore, a combination of theproposed feed-forward control technique and the traditionalfeedback-based control technique with a fast channel monitor may also beused to control Raman tilt transient under some circumstances withhigher performance requirement.

The gain medium used for the discrete RFA 300 may be conventional DCF102 as is shown in the above embodiments. It may also be other specialhigh nonlinear fibers such as high nonlinear photonic crystal fibers (orphotonic bandgap fibers) or a combination of conventional DCF 102 andother special high nonlinear fibers. In addition, the discrete DCF 102may be in a one-stage design as is illustrated in the above embodiments,it may also be in a multi-stage design, in which the Raman gain fiber isdivided into more than one stage separated by an isolator and each stagemay be either a forward-pumped RFA or a backward-pumped RFA. The Ramanpump may be a narrow-band Raman pump such as FBG-stabilized Fabry-Perot(F-P) laser, Raman fiber laser or multi-frequency distributed feedback(DFB) lasers. It may also be a broadband Raman pumps such as anincoherent Raman pump. For the case of using a broadband Raman pump, afast spectral filter may be required to perform fast output Raman pumpspectrum control.

The basic idea of this invention is also applicable to the commonEDFA/Raman hybrid-systems and even some all Raman systems using onlybackward-pumped RFAs at each span. In accordance with the presentinvention, very fast tilt transients due to inter-channel SRS effectsfrom multiple spans of transmission fibers and DCFs in these WDM systemsmay be controlled by using a forward-pumped RFA, which may be introducedby replacing or partly replacing an EDFA/a backward-pumped RFA. Thepresent invention is also applicable to an optical transmission systemusing other rare-earth-doped fiber amplifier (e.g. praseodymium-dopedoptical amplifier) as well as semiconductor optical amplifier andoptical parametric amplifier.

As described above, in a conventional hybrid EDFA/Raman long haulsystem, the proximity of a hybrid span in which transient tilt controlis provided to an ROADM 103 may vary according to system designcriteria. SRS tilt may depend on the types of transmission fiber used,the input signal power, channel patterns and a host of other factors.Permissible accumulated tilt before tilt compensation is useful maydepend on any of these factors and, in addition, the total number ofspans and margin allocation. Referring to FIG. 10, four exemplary longhaul systems are depicted in block schematic diagram format. In FIGS.10( a)-10(d), only two ROADM's 103-1 amd 103-2 are shown in exemplaryeight span WDM hybrid systems. It has already been suggested that amiddle span be selected for Raman transient tilt control. FIG. 10(a)shows an EDFA/Raman span with transient tilt control according to FIG. 3or 4-9 as the second span following the ROADM 103-1 or the second span(span 6) following the ROADM 103-2. Alternatively, the EDFA/RFA span isshown in FIG. 10( b) as immediately following the ROADM's 103-1 or 103-2as spans 1 and 5 respectively. FIG. 10( c) shows ROADM's 103-1 and 103-2separated by eight spans. In this case, transient tilt control and theEDFA/Raman spans are shown as spans two and six. In a similar case shownin FIG. 10( d), hybrid EDFA/Raman spans with transient tilt control areseen as spans one and five following ROADM's 103-1 and 103-2respectively. Other alternative long haul system designs are possiblewithin the spirit of the invention without limitation except to when itbecomes desirable to compensate for transient or static tilt, forexample, every fourth span in a hybrid or even in some Raman onlysystems.

As discussed herein, an EDFA/Raman hybrid system (or an all Ramansystem) can achieve both cost-effective optical transmission as well asfast tilt transient control, enabled by a simple feed-forward controltechnique. In the embodiments of FIGS. 8 and 9, feedback may be utilizedin place of feed-forward. Moreover, both forward and backward-pumpedRFA's may be used with a tilt control circuit in accordance with theinvention. In the presently preferred system, hybrid EDFA and RFA areused in only a small number of spans while EDFA's are used as thepredominant optical amplifiers in most of the spans. Because each RFA isused to control the tilt transients generated from multiple spans andthe introduced RFA not only performs the function of the replaced EDFAas an optical amplifier but also performs dynamic gain equalizationfunctions, a long haul fiber optic transmission system may be morecost-effective than the prior art by adding stand-alone dynamic tiltcompensators into a EDFA-only WDM system. The basic idea of thisinvention is also applicable to the common EDFA/Raman hybrid systems andeven to some all Raman systems where only backward-pumped RFAs are usedat each span to enhance Raman tilt transient control speed. Theinvention has been described with reference to a number of embodimentsand variations described of each embodiment. The invention should onlybe deemed to be limited in scope by the claims which follow.

1. An apparatus comprising: a Raman fiber amplifier; an associatedfeed-forward control circuit; an optical signal coupler configured tomonitor optical signal power and output a signal to the associatedfeed-forward control circuit, the feed-forward control circuitconfigured to compensate for a transient event introduced into anoptical fiber transmission system according to a gain control equation,wherein the associated feed-forward control circuit includes anadjustable speed of operation.
 2. The apparatus as recited in claim 1,wherein the optical signal includes an optical signal monitor configuredto monitor said optical signal power at two different wavelengths. 3.The apparatus as recited in claim 1, wherein the Raman fiber amplifieris a forward-pumped Raman fiber amplifier.
 4. The apparatus as recitedin claim 1, wherein the Raman fiber amplifier is a backward-pumped Ramanfiber amplifier.
 5. The apparatus as recited in claim 1, furthercomprising a fast spectrum monitor responsive to the optical signalcoupler.
 6. The apparatus as recited in claim 2, further comprising: aslow spectrum monitor for transmitting a control signal via an opticalsupervisory channel to a second control unit, wherein the associatedfeed-forward control circuit includes a first control unit responsive tosaid optical signal coupler and the second control unit.
 7. Theapparatus as recited in claim 6, wherein the slow spectrum monitor isconfigured to permit automatic measurement of a linear controlcoefficient for the associated feed-forward control circuit.
 8. Theapparatus as recited in claim 5, further comprising a slow spectrummonitor configured to provide a control signal to the associatedfeed-forward control circuit for the Raman fiber amplifier.
 9. Theapparatus as recited in claim 5, wherein the Raman fiber amplifier is abackward-pumped Raman fiber amplifier.
 10. The apparatus as recited inclaim 1, wherein the associated feed-forward control circuit, during atransient event, is configured to adjust power of at least one pump ofthe Raman fiber amplifier according to a linear equation.
 11. Theapparatus as recited in claim 10, wherein the linear equation comprises:${P_{L}( {j,t} )} \approx {{P_{L\; 0}(j)} + {\sum\limits_{k = 1}^{K}{{T_{LL}( {j,k} )}\lbrack {{S_{L}( {k,{t - T}} )} - {S_{L\; 0}(k)}} \rbrack}}}$where P_(L)(j,t) denotes a required pump power in a linear unit of thej^(th) pump at time instant t and S_(L)(k,t) denotes a detected inputsignal power in the k^(th) wavelength region and T represents acalculated propagation time between a point of detection of thetransient event and a resolution of the transient event.
 12. Anapparatus comprising: a Raman fiber amplifier; an associated feedbackcontrol circuit configured to control an amplifier gain by monitoringoptical signal power and to compensate for a transient event introducedinto an optical fiber transmission system; and an optical signal couplersystem configured to monitor optical signal power, the optical signalcoupler system including a fast channel monitor and a slow channelmonitor, wherein the associated feed-forward control circuit includingan adjustable speed of operation.
 13. The apparatus as recited in claim12, wherein the Raman fiber amplifier is a forward-pumped Raman fiberamplifier.
 14. The apparatus as recited in claim 12, wherein the Ramanfiber amplifier is a backward-pumped Raman fiber amplifier.
 15. Theapparatus as recited in claim 12, wherein the associated feedbackcontrol circuit utilizes a proportional-integration-derivative(PID)-based control algorithm.
 16. The apparatus as recited in claim 12,further comprising a length of dispersion compensation fiber, whereintime between detection of the transient event and its resolution isdetermined by the propagation time of the dispersion compensation fiber.17. A multi-span, multi-channel optical fiber transmission systemcomprising: at least three spans including a span configured to insert aRaman fiber amplifier and an associated feed-forward control circuit;and an optical signal coupler configured to monitor optical signal powerand output a signal to the associated feed-forward control circuit,wherein the associated feed-forward control circuit is configured tocompensate for a transient event introduced into the multi-channeloptical fiber transmission system, wherein the span configured to insertthe Raman fiber amplifier and the associated control circuit includes amiddle span of the at least three spans.
 18. The multi-span,multi-channel optical fiber transmission system as recited in claim 17,further comprising a plurality of Raman amplifiers among multiple spansincluding amplifiers.
 19. The multi-span, multi-channel optical fibertransmission system as recited in claim 17, further comprising at leastone rare earth doped fiber optic amplifier in a majority of the spans.20. The multi-span, multi-channel optical fiber transmission system asrecited in claim 17, wherein two spans of the at least three spansincludes a length of dispersion compensation fiber.